A relevant friction clutch is utilized in drivetrains between an internal combustion engine and a transmission, and controls the torque transmissible from the internal combustion engine to the transmission. If the transmission is designed as a dual-clutch transmission, then a friction clutch of this type is situated between each sub-transmission of the dual-clutch transmission and the internal combustion engine. When the friction clutch is in the engaged state, it transmits the maximum transmissible torque, including the torque peaks of the internal combustion engine which occur as a result of its rotational irregularities. In corresponding control methods of such friction clutches automated by means of a clutch actuator, these friction clutches are not engaged at the maximum torque, but are operated at a specified slip dependent on the driving situation, at which a rotational speed difference is regulated between the input part, which is connected to the crankshaft, and the output part of the friction clutch, which is connected to a transmission input shaft of the transmission, at which speed difference the torque peaks of the combustion engine are not transmitted, or only partially transmitted. This causes a damping of the torsional vibrations that occur in the drivetrain. In this connection, an unambiguous assignment of a slip condition and an adhesion condition is necessary in the control system of the friction clutch and in the transmission, for example, for adaptation processes of the friction clutch and shifting points of the transmission. To differentiate between a slip and an adhesion condition, a minimum slip with rotational speed differences between the input and output part of, for example, less than 50 revolutions per minute (rpm) is recognized as the transition from a slip condition to an adhesion condition. Such slight rotational speed differences are advantageous on the one hand for damping torque peaks, but on the other hand are overlaid by noise and vibration disturbances, so that detection of an adhesion condition is difficult.
German Patent No. 10 2009 029 741 A1 shows the adhesion condition of a friction clutch on the basis of a command variable that is traced back to the existing clutch torque.